Ohmic contacts for gallium arsenide

ABSTRACT

Improved contact resistance to a gallium arsenide body is obtained by alloying the contact metal into the GaAs body in an arsenic vapor atmosphere.

United States Patent 51 Mar. 7, 1972 King et a1.

OHMIC CONTACTS FOR GALLIUM ARSENIDE Inventors: George King; John William Frederick Rayner, both of Harlow; Anthony Charles Powell, Dunmow, all of England Assignee: International Standard Electric Corporation, New York, NY.

Filed: June 5, 1970 App1.N0.: 43,871

Foreign Application Priority Data Aug. 1, 1969 Great Britain ..38,730/69 US. Cl. ..1l7/227, 117/71 R, 117/106 R,

1 17/107, 117/217 Int. Cl ..C23b 5/50, C230 1/04, C230 13/04 Field oiSearch ..117/2l7,227, 107,71 R, 106A Primary Examiner-Alfred L. Leavitt Assistant Examiner-Kenneth P. Glynn AnomeyC. Cornell Remsen, Jr., Walter J. Baum, Paul W. l-lemminger, Charles L. Johnson, Philip M. Bolton, Isidore Togut, Edward Goldberg and Menotti J. Lombardi, Jr.

[57] ABSTRACT Improved contact resistance to a gallium arsenide body is obtained by alloying the contact metal into the GaAs body in an arsenic vapor atmosphere.

7 Claims, 7 Drawing Figures 1 Reva/'56 (u/went 0 Fave/we 19/05 Vo/foyp OHMIC CONTACTS FOR GALLIUM ARSENIDE BACKGROUND OF THE INVENTION The invention relates to a method of providing ohmic contacts on gallium arsenide and to devices produced by the method.

It is known that ohmic contacts can be formed on gallium arsenide by alloying a preform of pure tin to a surface of the gallium arsenide and this method results in devices with fairly high-resistance ohmic contacts and a relatively short operating life. Other forms of metallic contacts have also been used, for example: gold/germanium alloys, silver/tin alloys andindi- Virtually all the metallic contacts that have been used in the past suffer the disadvantage that during the formation of the contacts, gallium arsenide is dissolved into the metallic contacts and the region or regions of the gallium arsenide immediately under the metallic contacts becomes deficient in arsenic, thereby, resulting in fairly high-resistance ohmic contacts whose resistance value is difficult to predict. The electrical characteristics of devices produced in this manner are asymmetrical and are therefore liable to breakdown and have relatively poor efficiency.

SUMMARY OF THE INVENTION It is an object of this invention to provide for an improved method of providing ohmic contacts to semiconductor devices.

The invention provides a method of providing ohmic contacts on gallium arsenide including the steps of depositing the contact material on a surface of the gallium arsenide, and alloying the contact material to the gallium arsenide surface in the presence of arsenic vapor.

BRIEF DESCRIPTION OF THE DRAWINGS FIGS. 1 and 2 show steps in the method according to the invention.

FIG. 3 shows diagrammatically a sectional side elevation of an electric furnace utilized in the method according to the invention.

FIG. 4 shows a voltage/current characteristic for a Gunn Effect oscillator having its ohmic contacts formed by the method according to the invention.

FIG. 5 shows a voltage/current characteristic for a Gunn Effect oscillator having its ohmic contacts formed in the absence of arsenic vapor.

BRIEF DESCRIPTION OF THE PREFERRED EMBODIMENTS In the method according to the invention, the starting material is a body 1 of gallium arsenide which is shown in FIGS. IA and 1B of the drawings.

The first step of the method involves cleaning the body 1 and this is effected in several stages. The body is first of all boiled in trichlorethylene and any grease such as wax that remains after the boiling operation is removed by swabbing the body with hot trichlorethylene using for example a clean cotton bud. The body is next boiled in methanol and then in isopropylalcohol. Any traces of white residue remaining on the body after these boiling operations is removed by swabbing the body with hot trichlorethylene using for example a clean cotton bud.

The next step of the method involves the deposition of metal contact layers on say the surfaces 2 and 3 of the body I. This step is effected by evaporating or sputtering a metal, for example a silver/tin alloy onto the surfaces 2 and 3 to form the layers 4 and 5 on the body I as shown in FIGS. 2A and 2B of the drawings. The layers 4 and 5 are then alloyed to the body 1 in a partial pressure of arsenic vapor to form low-resistance ohmic contacts.

The evaporation of the silver/tin alloy layers 4 and 5 is effected in a vacuum at a pressure less than l" TORR. Each of the layers 4 and 5 is preferably formed by firstly evaporating a layer of tin on the appropriate surface of the body I and then evaporating a layer of silver on the surface of the layer of tin. In a practical arrangement, the layer of tin is 800 A thick and the layer of silver is 1,100 A thick.

The alloying of the layers 4 and 510 the gallium arsenide body 1 to form low-resistance ohmic contacts is effected in say an electric furnace 11 in a manner as shown diagrammatically in FIG. 3 of the drawings. The body 1 with the layers 4 and 5 thereon which is indicated in the drawing by the reference 6, is mounted in a boat 7 in the furnace tube 8 and the arsenic metal 9 is mounted in a spectrosil boat 10 in the tube 8. Due to the rapid temperature gradient in the tube 8, the temperature of various parts of the arsenic metal sourceis between 350 and 400 C. The temperature within the tube 8 including the region where the boat 7 is positioned is 610 C. and hydrogen (H) gas is passed through the tube 8 during the alloying process. Alloying, therefore, takes place at a temperature of 6l0 C. in a partial pressure of arsenic vapor. The layers 4 and 5 are alloyed for a period of 5 minutes and the device 6 is allowed to cool before it is removed from the f$rnace. In a practical arrangement, the tube 8 is 2.5 cm. in diameter and the hydrogen gas flow rate is cc. per minute.

A typical gallium arsenide device that can be produced by this method is a Gunn Effect oscillator which is basically an active region of gallium arsenide i.e., a solid piece of material or an epitaxially deposited layer on a surface of a substrate, having spaced ohmic contacts associated therewith. The requirements of a Gunn Effect oscillator are that the active gallium arsenide region or body must be of very high purity material and of the correct resistivity and that low-resistance ohmic contacts are required for the oscillators.

The method according to the invention has resulted in Gunn Effect oscillators with improved efficiency and has greatly reduced the risk of arsenic losses in the alloying step. 7

When the ohmic contacts of a Gunn Effect oscillator are made by the method according to the invention, the gallium arsenide active region of the oscillator can be bonded to a heat sink by means of one of the contacts without any degradation and the operating life of the oscillator is considerably increased. I

An example of the voltage/current characteristics for the same piece of gallium arsenide alloyed with and without ar senic are respectively shown in FIGS. 4 and 5 of the drawings. The oscillator having the characteristic of FIG. 4 oscillates well for both polarities of bias voltage and gives higher efficiencies than the oscillator having the characteristic of FIG. 5;

these oscillators (FIG. 5) invariably burn out when reverse biased.

In Gunn Effect devices in which the active region of the gallium arsenide is situated between two layers of highly doped gallium arsenide, the metal-semiconductor ohmic contacts are required on the highly doped gallium arsenide layers. The contacting techniques outlined in preceding paragraphs can still be utilized to advantage although the additional gallium arsenide layers tend to increase the thermal resistance between the active region and the heat sink, and the improvement in efficiency of the devices may not be so great.

It is to be understood that the foregoing description of specific examples of this invention ismade by way of example only and is not to be considered as a limitation on its scope.

What is claimed:

1. A method of forming ohmic contacts on a gallium arsenide body comprising the steps of:

depositing a silver tin layer on at least one surface of said body; and

alloying said layer into said one surface in the presence of arsenic vapor, said one surface being at a temperature of approximately 610 C. 4

2. A method according to claim 1, further comprising:

depositing another silver tin layer onto another surface of said body; and

alloying said other layer into said another surface in the presence of arsenic vapor.

said arsenic vapor is provided by flowing a gas over an arsenic metal source and passing said gas over said body.

6. A method according to claim 5, wherein said arsenic metal source is heated to approximately 350 to 400C.

7. A method according to claim 5, wherein said gas is hydrogen haVing a flow rate over said body of approximately cc./min. 

2. A method according to claim 1, further comprising: depositing another silver tin layer onto another surface of said body; and alloying said other layer into said another surface in the presence of arsenic vapor.
 3. A method according to claim 1, wherein said silver tin layer is evaporated onto said one surface at a pressure less than 10 5 TORR.
 4. A method according to claim 1, wherein said silver tin layer is deposited by first evaporating a layer of tin onto said one surface and then evaporating a layer of silver onto said tin layer.
 5. A method according to claim 1, wherein the presence of said arsenic vapor is provided by flowing a gas over an arsenic metal source and passing said gas over said body.
 6. A method according to claim 5, wherein said arsenic metal source is heated to approximately 350* to 400* C.
 7. A method according to claim 5, wherein said gas is hydrogen haVing a flow rate over said body of approximately 85 cc./min. 